Sulfur is one of elements which take important roles in nature. Particularly, it takes a substantial role as a constituting element for cysteine and methionine as sulfur-containing amino acids. Further, it has been known that a large sulfur cycle is carried out between plants and animals. In the plants, sulfur is taken in the form of sulfate ions and reduced to sulfide ions, and then subjected to cysteine synthesis and further methionine synthesis. In the animals, methionine is ingested from foods by food chain and metabolized to cysteine in vivo. In this metabolization stage, homocysteine is formed as an intermediate.
Homocysteine is an intermediary metabolite which is scarcely present at the normal condition. However, it has been reported that when its concentration in blood is at a high level, the rate of occurrence of coronary disease and cerebral apoplexy is high. Accordingly, the homocysteine amount in blood is already recognized or still unrecognized to be a risk factor useful for predicting the occurrence of thromboembolism such as myocardial infarction or cerebral infarction, or arteriosclerosis.
Further, since cysteine is an amino acid formed by metabolism of methionine, it may be an auxiliary index for grasping the causes of metabolic error of homocysteine.
Furthermore, enzymes are known which have a function of acting on homocysteine or cysteine and conducting decomposition or substitution, thereby forming hydrogen sulfide. However, even if it is attempted to quantitatively determine homocysteine or cysteine by utilizing such enzymes, since there is no method for measuring the formed hydrogen sulfide conveniently with high sensitivity, no practical method has been found in which these enzymes are used for quantitatively determining homocysteine or cysteine.
On the other hand, hydrogen sulfide or sulfide ions derived therefrom are important as an index of environmental pollution such as air pollution or water pollution of rivers. The state of pollution can be confirmed by measuring the hydrogen sulfide or sulfide ions derived therefrom in samples.
As conventional methods for measuring hydrogen sulfide or sulfide ions derived therefrom, the following methods are known. For example, as a method of utilizing a color developer, a method wherein 2,2′-dipyridyldisulfide (Svenson, Anal. Biochem., 107; 51–55(1980)) or sodium nitroprusside is used, a method wherein N,N-dimethyl-P-phenylenediamine and ferric chloride are reacted in a strong acidic condition to form methylene blue and blue color development is detected (methylene blue method), a method wherein fading amount and rate of a dye (toluidine blue or methylene blue) is measured by using cerenium as a catalyst (Mousavi et al, Bull. Chem. Soc. Jpn, 65;2770–2772(1992), Gokmen et al, Analyst, 119;703–708 (1994)), and the like are known. However, these methods are hardly adequate from the viewpoints of convenience and sensitivity.